RESULTS FROM THE BO LIQUID ARGON SCINTILLATION TEST STAND AT FERMILAB - - PowerPoint PPT Presentation

RESULTS FROM THE BO LIQUID ARGON SCINTILLATION TEST STAND AT FERMILAB

Ben Jones, MIT

New Perspectives Fermilab, June 11th 2013

Bo VST Setup

• Bo Vertical Slice Test is a training

ground for one slice of the MicroBooNE optical system including:

• Cryogenic photomultiplier tubes
• Base electronics
• Wavelength shifting plate
• High voltage system + interlocks
• Cables and splitters
• Readout electronics
• Cryostat feedthrough
• Trace impurity monitors
• Etc…
• But also a fantastic R&D detector for

studying liquid argon scintillation light uB style PMT assembly

Experimental Configuration for This Study

4

Prompt peak window

Light in Liquid Argon

• The scintillation light in liquid argon is produced copiously alongside all

ionization charge deposits.

• There are two scintillation pathways, with different time constants – a fast

component with t=6ns and a slow time constant with t=1500ns. Ar Ar p + Ar Ar

* *

1Σu excimer

Ar Ar γ 6ns e Ar Ar +

• Ar

p + Ar e - + Ar Ar *

3Σu excimer

1590ns

Special bonus – possible PID information

Ar Ar * Ar Ar * Ar Ar Ar Ar Ar Ar * Ar Ar γ

• Utlized in dark matter searches (MiniCLEAN, DEAP), and we are investigating the

applications of this technique to augment TPC based particle ID in MicroBooNE.

Scintillation process Competing Excimer Dissociation Process

Pulse shape discrimination – a vital tool in dark matter detection, also useful to us!

Individual components (separated using PSD) Fit function for alpha + background

The Effects of Nitrogen in Argon

• Unlike oxygen and water, nitrogen

does not disturb charge drift in LArTPCs, and is difficult to remove from argon.

• Part per million (ppm) levels of

dissolved nitrogen are expected to be present in any large future LArTPC detector

• Nitrogen at the ppm level leads to :
• 1) Scintillation Quenching

measured in a detailed study by the WArP collaboration in small test cells

(R Acciarri et al 2010 JINST 5 P06003)

• 2) Absorption of Scintillation Light

Absorption effects of N2 in LAr have not previously been measured

(late light lifetime is affected by N2 – So can’t use PSD for this study)

Our Paper (coming soon…)

General Idea:

• Source set in one of two possible

positions.

• Controlled amounts of N2 injected into

the liquid

• Quenching affects both source positions

equally

• Absorption hinders the further more than

the nearer source.

• If fractional losses from each source

deviate we see an N2 absorption length effect.

• A future analysis will address the effects
• f quenching (more extensively studied

by other groups) separately.

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14.5”

• PPM amounts of nitrogen are injected into

the liquid from a gas canister, charged to a known pressure.

• From known volume of canister and

known pressure we can calculate how many ppm we injected.

• Nitrogen concentration monitored in both

liquid and gas phases using LDetek8000 N2 monitor

• We also monitor H20 and O2 to ~10ppb

precision from the same sample lines.

Trace nitrogen monitor Injection Canister

Kindly loaned by Jong Hee Yoo – Thanks!

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Light loss due to N2 in 8” source configuration 27ppb N2 3.7ppm N2 7.4ppm N2 15.5 ppm N2 Preliminary

Attenuation Data

Preliminary Divergence of these two lines is clear evidence for the nitrogen absorption effect!

Stability of 1PE

• SPE scale stable

to within 1% for each run

• This is similar to

the precision of

• ur SPE

measurements

• Therefore we

assume constant and fold in variations as a systematic error

• n each point

Just to be sure its really the nitrogen…

Preliminary No light loss during periods with no nitrogen injection – gives confidence in system stability, constrains

• utgassing effects, etc.

Getting to the Attenuation Strength

Measured Attenuation Strength: Measured Absorption Cross Section:

Preliminary

Comparison to N2 gas absorption cross section world data

Preliminary

Nice result, but whats it gonna do for me?

$$$$ $

Preliminary

Summary + Prospects

• Bo VST has been constructed to test elements of MicroBooNE
• ptical system – also an R&D detector for LAr scintillation light.
• Detailed studies of alpha source response have been made

and area used in various Bo VST studies

• We have measured the effects of nitrogen absorption of 128nm

argon scintillation light in liquid argon. We find that the effect is

• n the order 0.015% / (ppm cm)
• This means absorption is no problem for MicroBooNE, and

could be useful information for the design of cryo systems for large LArTPCs

Backup Slides

Understanding the Geometrical Effect

Ray trace to understand expected light yields per percent

• f absorption at each position

8” 14.5”

Taking ratio, any quenching effect cancels

Ratio = Light loss at 8” Light loss at 14.5”

Our region of interest

We will measure the nitrogen absorption effect as % light loss per ppm^-1 cm^-1. First, measure the light loss ratio as a function of N2 concentration. In our region of interest the relationship should be ~linear. Absorption strength extracted by comparing the gradient of the measured line to the gradient of the line right, which gives proportionality factor for X axis scales. This factor tells us the % light loss per ppm cm of nitrogen.

5 10 15 20 25 30 35 40 45 50 55 60 65 70 5 10 15 20 25

Measured Gas Concentration (ppm) Measured Liquid Concentration (ppm)

Air Liquide Saturation Tables NIST REFPROP (Tope)

2) Measurement from liquid and gas capillaries in agreement with saturation pressure based equilibrium calculation 1) Amount of N2 in liquid agrees with amount injected to within our uncertainty of the injection volume.

How do we know we get N2 concentration right?

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Single exponent power law (cosmic background) + Poisson (alpha source) Detected light spectrum – clean argon, source at 8”

Check on functional form of fits: Power law background is great. Alpha fit needs improvement (not exactly poissonian).

Why?

“Shadowing” of outer source edges leads to reduced poisson mean light yield from edge area elements This leads to an enhanced low tail of the source spectrum

Disc source kindly loaned by Adam Para – Thanks!

So we Measure the Shadowing Function…

Now we know how the source is shadowed, we know how to fit all points.

Improved fit from shadowing function

Major improvement with new fit function. Note : no extra free parameters, since shadowing function was tuned on an independent dataset.

Aside: Pulse Shape Discrimination in Action

Alpha enriched Cosmic

• nly

Saturation

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PMT Characterizations for MicroBooNE

• Measured dark rat

128nm 1.18 ± 0.1 Visible photons out / UV photon in for evaporative TPB

Gehman et al

35

Expected Light Yield at Plate

36

More ray tracing, should be straightforward enough…

• Nope

1” 6 mm 1/8” Side view Top view

?

Obscured by holder

37

Baseline Less Obscured More Obscured Try a few options; System has cylindrical symmetry, so distribution in phi does not matter.

0 < r < 1.5mm : Empty 1.5 < r < 3mm : Uniform source 0 < r < 3mm : Uniform Source 0 < r < 1.5mm : Uniform source 1.5 < r < 3mm : Empty

¼ of plate area covered ¾ of plate area covered Full plate area covered

38

Propagation Effects – Impurity Absorption

• No theoretically known absorption mechanism at 128nm in pure

argon

• But ~ppm impurities can lead to finite absorption lengths.
• For this test we monitored O2, N2 and H20 at <10ppb precision

* *

* = First installation and test of actual MicroBooNE cryo analytics!

39

SPE Insensitivity